1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Determining the final position of test charge with multiple potentials

  1. Jan 23, 2009 #1
    1. The problem statement, all variables and given/known data

    Consider the following arrangement of potentials and a test charge (not pictured) located at infinity. - To which location will a test charge of +3 microColumbs travel?

    The potentials are as follows:
    • (-1V) top left
    • (-7V) top right
    • (+1V) lower left
    • (+4v) lower right

    They do not line up(top left is further left than bottom left, etc.), and no distances are given




    2. Relevant equations

    force = q * E (charge * field strength)
    All of the other equations I can think of require distance


    3. The attempt at a solution

    I honestly don't know what to do here. If I was given distances I would be able to calculate the force on the charge, but since I am not I do not know how to calculate where exactly it will end up. I assume it will be near the -7V charge, but that is a very general answer. If someone could shed a little light on how I could figure this out I'd greatly appreciate it
    Thanks!
     
    Last edited: Jan 23, 2009
  2. jcsd
  3. Jan 23, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    Welcome to PF.

    Isn't it a matter of which Voltage node the charge will be most attracted to if left to follow the field lines on its own?
     
  4. Jan 24, 2009 #3
    Thanks, it looks like a great place!

    I was thinking that at first, but I noticed that it is a 4 point question, so I was guessing that there had to be something more to it. I don't know what else you could do with the info given though. If that was what they were looking for, it would be the -7V charge, correct?
     
  5. Jan 24, 2009 #4

    LowlyPion

    User Avatar
    Homework Helper

    Looks like it to me too.

    A +3 would mostly likely want to go steady with the -7.
     
  6. Jan 25, 2009 #5
    The next part of the question asks about the kinetic energy when it arrives. I would only have to use the -7V potential to calculate that and not any of the others, correct?
     
  7. Jan 25, 2009 #6

    LowlyPion

    User Avatar
    Homework Helper

    From ∞ the net voltage is what determines the work to get there. Hence the sum of the voltages. I think the energy when it arrives will be influenced by the net of the voltages.
     
  8. Jan 26, 2009 #7
    Sorry, one last question. It also wants me to find the location with a -3 microColumb test charge. Would this travel to the +4V potential even though the net charge is negative? I have to calculate its potential energy for the next part, but I should be ok for that as long as I know where it is going. I apologize for all of the questions but I want to understand this and it is not covered at all in our lessons.
     
  9. Jan 26, 2009 #8

    LowlyPion

    User Avatar
    Homework Helper

    If the test charge is (-), and the net voltages are (-), what incentive does it have to leave ∞?

    If it doesn't leave ∞, what change in potential energy is there from 0 that would be possible.

    (If you say it gains - potential then I will wonder if your charge is really at ∞.)
     
  10. Jan 26, 2009 #9
    I figured it would just remain at infinity also, but the next part gives the mass of the new test charge and asks for its speed when it reaches its final destination. If both the charge and net potential acting on it are negative and the charge is already at infinity, how can you calculate its speed? I'll type everything out for you and see what you think:

    A. To which location will a test charge of +3 microColumbs travel
    B. What will be its kinetic energy when it arrives
    C. Find the capacitance of a capacitor that would store the same amount of energy as you found in part B
    D. suppose the +3 microColumb charge is replaced with a test charge of -3 microColumbs. To which location will it travel?
    E. If the new test charge has a mass of 27.3E-21 kg, what will be its speed when it arrives.

    My only thought is that maybe they are talking about switching the test charge after it has been pulled in from infinity to the -7V potential. What do you thinK?

    I'll note quickly that this is an AP Physics online course and some of the homework problems have proven to be literally impossible...it can be frustrating.
     
    Last edited: Jan 26, 2009
  11. Jan 26, 2009 #10

    LowlyPion

    User Avatar
    Homework Helper

    When it arrives - from ∞ - its potential energy would be converted to kinetic wouldn't it?

    ΔPE = ½mv²

    But if they are talking about the -3 test charge I think ΔPE = 0.
     
  12. Jan 26, 2009 #11

    Right. The thing is, the net potential is -3V. The charge is now -3uC. There is no reason that it would leave infinity, correct?
     
  13. Jan 26, 2009 #12

    LowlyPion

    User Avatar
    Homework Helper

    Yes, I just added that thought to the last post.
     
  14. Jan 26, 2009 #13
    ah...got it. Well, I'm just going to assume that it is starting from where the +3uC charge was and going to infinity from there since I don't know what else I can do. Thanks for all of your help! I'm not used to asking questions, but I was lost on this one.
     
  15. Jan 26, 2009 #14

    LowlyPion

    User Avatar
    Homework Helper

    You might want to check again to be sure the signs of the problem are correct as stated. But I think with the current statement, if the -3 is at ∞ it stays there if the net of the voltages is (-).
     
  16. Jan 26, 2009 #15
    I just double checked, the signs are correct. Since it gives me a mass and information, I'm sure that it wants a speed calculated, which means that the -3 has to move(it asks for the speed of the new test charge). From that, I'm just guessing that when they talk about changing charges in D they are replacing the +3 with -3 after it has come all the way in to the potentials. The -3 would then be repelled out to infinity. Think that could be right?
     
  17. Jan 26, 2009 #16

    LowlyPion

    User Avatar
    Homework Helper

    No. If they swap the charges, that means put a -3 at the -7 voltage, then it would arrive at the +4 voltage node having experienced an acceleration of 7v + 4v or 11 v.

    This charge if it were located at the -7 would go to the +4 directly, no visit to ∞ involved. As I originally read what you posted I thought it meant start at ∞ again.

    Edit: As before ΔKE = q*ΔV = ½mv²
     
    Last edited: Jan 26, 2009
  18. Jan 26, 2009 #17
    Of course...I forgot about that. What do you think I should use for calculating the speed then? I know it won't have a speed of 0(they wouldn't make it a 4pt question or give mass), so it has to be moving somewhere.
     
  19. Feb 11, 2009 #18
    I'm having trouble figuring out how you solved for this problem. I understand that the kinetic energy would be equal to the potential energy, but I'm not sure how to solve for the potential energy :eek:
     
  20. Feb 11, 2009 #19

    LowlyPion

    User Avatar
    Homework Helper

    ΔPE = W = q*ΔV
     
  21. Feb 11, 2009 #20
    But the velocity is not given, is it?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Determining the final position of test charge with multiple potentials
Loading...